1. Field of the Invention
The present invention relates to an electron beam-excited blue phosphor.
2. Description of the Related Art
An image display apparatus utilizing light emission by electron beam excitation is a display device that provides a display apparatus which: is of a self-luminous type; has good color reproducibility and high luminance; and is excellent in dynamic image displaying property, and the image display apparatus has been put into practical use as a cathode ray tube (hereinafter, referred to as “CRT”) since olden times.
Meanwhile, additional improvements in performance, size, and image quality have been requested of the image display apparatus in association with recent diversification and recent increase in density, of image information. In addition, a demand for a flat panel display (FPD) has been remarkably growing in recent years in association with an increase in needs of the time, such as energy savings and space savings.
In addition, a thin-film transistor-driven liquid crystal display device (TFT-LCD), a plasma display module (PDP) utilizing vacuum ultraviolet emission by plasma, and the like have already been put into practical use, and have replaced part of the market conventionally occupied by the CRT. However, the TFT-LCD has the following problem: bad dynamic image visibility resulting from a narrow view angle and bad responsiveness. In addition, the PDP has the following problems: insufficient luminance, bad responsiveness, and high power consumption. Any one of those problems may be fatal to a widespread use of the TFT-LCD or the PDP as a display that will completely replace the CRT in the future.
On the other hand, a field emission display (hereinafter, referred to as “FED”) as a flat image display apparatus utilizing electron beam excitation has the following characteristics: the FED can respond at a high speed, has high luminance, and consumes small power. Accordingly, expectations have been placed on a commercial use of the FED in order that a full-fledged spread of the FPD may be achieved.
An outline of a structure of the FED is as described below. That is, devices each of which can serve as an electron source are placed on a rear plate in a matrix fashion in correspondence with respective pixels, and wiring necessary for driving these many devices is formed in a matrix fashion. A large number of forms have been proposed for the “electron source” in this case, and examples of the forms include an electron source belonging to a cathode tip having a three-dimensional structure, such as a so-called Spindt type electron source, a flat electron source, and an electron source using a carbon nanotube. An application of a voltage corresponding to image information to the electron sources through the wiring in a vacuum causes electrons corresponding to the image information to be emitted like beams.
In addition, a face plate has a layer formed of a phosphor that emits light by using an accelerated electron beam as an excitation source. An application of a high voltage between the rear plate and the face plate accelerates electron beams emitted from the electron sources. The accelerated electron beams provide the phosphor with necessary excitation energy, whereby an image corresponding to the image information is formed.
In addition, in the face plate, charge accumulating on the layer of the phosphor that is substantially an insulating substance must be efficiently removed, and light emitted from the phosphor must be efficiently reflected. Accordingly, a film made of a metal having a small atomic number such as aluminum, which is called metal back, is generally formed on the layer of the phosphor. However, energy loss due to the metal film becomes significant in an FED to be used in a region where an acceleration voltage is low, so a conductive transparent film made of, for example, an indium tin oxide is formed on a face plate in some cases.
In addition, the FED is driven via the above mechanism, so a high-vacuum container having a degree of vacuum of about 10−4 Pa or more is needed. Accordingly, a frame having an appropriate thickness is inserted between the face plate and the rear plate before both the plates are bonded to each other. In addition, multiple parts each referred to as a spacer are placed between the face plate and the rear plate because the plates must be supported against atmospheric pressure. In addition, in general, the container is evacuated to be a vacuum container.
In ordinary cases, each spacer is placed between adjacent pixels of the phosphor, that is, on a black non-light-emitting region (black matrix) provided for suppressing reflection of external light. In addition, in general, the number of spacers to be placed should be sufficient for sufficiently support against the atmospheric pressure.
By the way, an interval between a cathode plate on an electron source side and the face plate as an anode in the FED characterized by its flatness is typically reduced to several millimeters, so an acceleration voltage of 25 kV or more cannot be applied unlike the CRT from the viewpoint of a withstand voltage. Accordingly, even an FED of a high-voltage type is said to be capable of withstanding an acceleration voltage of only about 15 kV or less.
As a result, a depth at which an excited electron penetrates into the layer of the phosphor cannot help being shallower than that in the case of the CRT, so adoption of, for example, a high current density or line-sequential driving is indispensable for realization of luminance needed for practical use, which is equivalent to that of the CRT.
The foregoing strongly requests, of the phosphor, not only realization of high luminous efficiency but also securement of luminance linearity in a high current region and stability of luminance against an input of charge. Further, the realization of a high-level display device requires the phosphor to show a luminescent color having high color purity.
By the way, electron beam-excited phosphors that can be currently put into practical use in terms of properties including luminous efficiency and a color purity are limited to a group of zinc sulfide phosphors each referred to as EIA name “P22”, the phosphors being adopted in most CRT's.
Such phosphor using zinc sulfide as a host material does not have sufficient stability against the input of charge. Accordingly, the luminance of such phosphor is expected to deteriorate remarkably in an FED to be operated in a higher current region than that in the case of the CRT.
In addition, various problems have been pointed out: a sulfur atom (S) dissociated by thermal energy generated by the input of charge scatters in a vacuum container to reduce the degree of vacuum of the container, and furthermore, adversely affects an electron source.
Further, the above problems are particularly notable in a blue phosphor “ZnS:Ag phosphor” that requires the highest current because one has difficulty in visually feeling luminous efficiency.
To solve the above problems, there have been proposed, for example, a method of producing a zinc sulfide phosphor having a small number of crystal defects as disclosed in JP 2002-265942 A and a step of correcting the crystal defect or surface strain layer of a zinc sulfide phosphor as disclosed in JP 2004-307869 A, and some degree of alleviating effects on the problems have been obtained.
However, a zinc sulfide phosphor has, in addition to a problem concerning lack of stability against the input of charge, the following problem: luminance linearity deteriorates in a high current region.
The problem is due to, for example, the following reasons: the mechanism via which the zinc sulfide phosphor emits light is a second order reaction referred to as a donor-acceptor pair light emission type, and a donor-acceptor concentration cannot be sufficiently increased due to problems such as concentration quenching.
As described above, development of an electron beam-excited blue phosphor that may replace the zinc sulfide phosphor has been strongly demanded.
Meanwhile, an improvement in stability against the input of charge with, for example, such surface protective film as disclosed in JP 2004-285363 A has also been proposed. In the proposal, a surface protective film made of, for example, a phosphate is used, and its applications are not limited to a zinc sulfide phosphor. However, according to an experiment conducted by the inventors of the present invention, the improving effect of the film on the stability was not sufficiently large, and a problem such as the introduction of adsorbed water into a vacuum container occurred.
A projection tube (PRT) has also been put into practical use as a cathode ray tube using a high current as in the case of the FED, but a zinc sulfide phosphor has been adopted for a blue color from the viewpoints of a color purity and luminous efficiency. There has been proposed, for example, a method of correcting a phenomenon called current luminance saturation on a driver circuit side as disclosed in JP 2971104 B or JP 02-186537 A. However, the method disclosed in each of the documents is a method involving increasing a display screen load, and is not preferable in terms of stability against the input of charge.
Proposals such as cooling of a tube surface have been proposed against problems concerning the deterioration of luminance in association with the input of charge and the spread of a spectral band width due to heat generation, and have been put into practical use. At present, however, the proposals have not sufficiently solved the problems yet.
Meanwhile, a search and investigation for a phosphor except zinc sulfide have also been conducted with a view to solving those problems. For example, a Y2SiO5:Ce3+ phosphor referred to as EIA name “P47” has been commercialized and adopted as a blue phosphor for a beam indexing tube belonging to a special cathode ray tube.
The P47 phosphor uses an oxide as a host material and is generally said to have higher stability against the input of charge than that of a zinc sulfide phosphor.
In addition, the mechanism via which Ce3+ emits light is quick first order attenuation in association with the allowed inner shell transition of 4f5d, and the phosphor is excellent in luminance linearity in a high current region because the phosphor has a high recycling rate of a luminescent center.
The fact that the phosphor is excellent in those properties can be actually confirmed by evaluation conducted by the proposers. However, the width of the emission spectrum of the phosphor cannot be narrowed because of, for example, the following reasons: the 5d splitting energy of Ce3+ in a Y2SiO5 crystal is not sufficiently high, and the splitting of an f orbital in association with a spin-orbit interaction is relatively large. Therefore, the development of any other phosphor host material is needed for obtaining a blue luminescent color having a sufficient purity.
On the other hand, there are some reports concerning the light-emitting property by electron beam excitation of a BaMgAl16O27:Eu phosphor for blue illumination known to show a high color purity and high luminance by vacuum ultraviolet light excitation. However, according to the reports, the phosphor deteriorates due to remarkable blackening as a result of irradiation with an electron beam, and causes a problem upon practical use.
In addition, the experiment conducted by the inventors of the present invention actually provided similar results: the host material of the phosphor started to color in association with irradiation with an electron beam, and hence the luminance of the phosphor reduced.
On the other hand, a report on a CaMgSi2O6:Eu phosphor similarly known to show a high color purity and high luminance by vacuum ultraviolet light excitation is limited to Appl. Phys. Lett., 72, 1998, 2226 by F. L. Zhang et al in which the phosphor is simply evaluated for behavior after electron beam excitation in a low current region. However, the experiment conducted by the inventors of the present invention has provided the following results: with regard to chromaticity, the CaMgSi2O6:Eu phosphor can provide a blue color having a higher purity than that in the case of the P22 phosphor, but the luminance and lifetime of the CaMgSi2O6:Eu phosphor are substantially comparable to those of the P22 phosphor. A mechanism different from that of zinc sulfide has been proposed for the problem concerning the lifetime.
That is, the mechanism is as follows: an oxygen vacancy is present in the surface of a phosphor, and an oxygen atom in a bulk responsible for the transfer of energy to a luminescent center moves toward the vacancy as a result of the input of excitation energy. JP 2002-348570 A discloses a method based on a reheat treatment in an oxygen atmosphere for solving the problem. However, the method has not been preferable because the luminescent center is apt to be prevented from emitting light due to the oxidation of a luminescent center metal ion Ce3+ or Eu2+.
As described above, the FED requires a blue phosphor which: shows sufficiently satisfactory color purity under low and medium acceleration voltages of about 15 kV or less; secures luminance linearity in a high current region; and stably emits light against the input of charge.